Deterrence/Prevention

Eliminating the intermediate host snail is the key to controlling infection with intestinal flukes.

Proper cleaning and processing of raw vegetables by immersing in boiling water for a few seconds, followed by peeling and washing in clean water, is important in preventing infection with intestinal flukes, especially in endemic areas.

Night soil (human excreta) and pig excreta should not be used as fertilizers.

Metacercariae are not necessarily inhibited or destroyed by interventions such as smoking or freezing; and the practice of fish pickling is ineffective. Irradiation practices may be of benefit in managing the burden of metacercariae in the process of food preparation.

Many strategies to control opisthorchiasis have been used in Thailand. The strategies include mass drug administration and education to decrease the consumption of raw fish. Over the last decade, these methods have changed the epidemiology of O viverrini infection in Thailand. In the past, O viverrini infections were characterized as heavy infections that were concentrated in particular villages. Now, because of these strategies, O viverrini infections have become "lighter" but more widespread in terms of geographic distribution.
[48]

Parasitic helminths reside in immunologically exposed extracellular locations within their hosts, yet they are capable of surviving for extended periods. To enable this survival, these parasites have developed complex and multifaceted mechanisms to subvert or suppress host immunity. There are studies on the immunomodulation by helminth parasites of ruminants and the parasite-derived molecules involved in driving this modulation. Such immunomodulatory molecules have considerable promise as vaccine targets.
[49]

Triclabendazole 10 mg/kg as a single dose is the preventive chemotherapy for fascioliasis and is recommended in subdistricts, villages, or communities with clustering of cases. The WHO recommends that all school-aged children (aged 5-14 years) or all residents should be given the preventive chemotherapy every 12 months.
[47]

Praziquantel 40 mg/kg as a single dose is the preventive chemotherapy for clonorchiasis and opisthorchiasis and is recommended in all people residing in areas where the prevalence of infection is 20% and above. Among people who live in areas with less than 20% prevalence, the recommendation is to administer preventive chemotherapy (1) to everyone every 24 months or (2) every 12 months to those who eat raw fish.
[47]

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Prognosis

Light infections may resolve spontaneously within one year, even without treatment. The prognosis may be grave in patients with heavy infection.

Immunocompromised hosts may be at an increased risk of complications. For example, G seoi worms were found to penetrate into colonic lymphoid tissue in a patient with colon cancer.
[50]

Life cycle of Fasciolopsis buski. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

The life cycle of Fasciolopsis. Immature eggs are discharged into the intestine and stool and become embryonated in water. The eggs then release miracidia, which invade a suitable snail intermediate host, in which the parasites undergo several developmental stages (sporocysts, rediae, cercariae). The cercariae are released from the snail and encyst as metacercariae on aquatic plants, which are eaten by mammalian hosts (humans and pigs), who become infected. After ingestion, the metacercariae excyst in the duodenum and attach to the intestinal wall, where they develop into adult flukes (20-75 mm X 8-20 mm) in approximately 3 months and attach to the intestinal wall of the mammalian hosts. The adults have a life span of about one year. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

Egg of Fasciolopsis buski. Images reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

Adult fluke of Fasciolopsis buski. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

The life cycle of Heterophyes. The adult parasites release embryonated eggs (each with a fully developed miracidium), which are then passed in the host's feces. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Snails of the genera Cerithidea and Pirenella are important hosts in Asia and the Middle East, respectively. The miracidia undergo several developmental stages in the snail (sporocysts, rediae, cercariae). Many cercariae are produced from each redia. The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater or brackish-water fish (second intermediate host). The definitive host becomes infected by ingesting undercooked or salted fish that contains metacercariae. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine, and mature into adults (measuring 1-1.7 mm X 0.3-0.4 mm). Heterophyes heterophyes infects humans, various fish-eating mammals (eg, cats, dogs), and birds. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

Life cycle of Metagonimus. The adult parasites release fully embryonated eggs (each with a fully developed miracidium), which are then passed in the host's feces. After ingestion by a suitable snail (first intermediate host), the eggs hatch and release miracidia, which penetrate the snail's intestine. Snails of the genus Semisulcospira are the most common intermediate host for Metagonimus yokogawai. The miracidia undergo several developmental stages in the snail (sporocysts, rediae, cercariae). Many cercariae are produced from each redia. The cercariae are released from the snail and encyst as metacercariae in the tissues of a suitable freshwater or brackish-water fish (second intermediate host). The definitive host becomes infected by ingesting undercooked or salted fish that contains metacercariae. After ingestion, the metacercariae excyst, attach to the mucosa of the small intestine, and mature into adults (measuring 1-2.5 mm X 0.4-0.75 mm). M yokogawai infects humans, fish-eating mammals (eg, cats, dogs), and birds. Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

Various animals may be definitive hosts for different Echinostoma species, such as aquatic birds, carnivores, rodents, and humans. Unembryonated eggs are passed in stool (1), and development occurs in the water (2). The miracidium takes an average of 10 days to mature and then hatches (3), penetrating the first intermediate host, a snail (4). Snails, in general, serve as the first intermediate host. The intramolluscan stages are as follows: sporocyst (4a); rediae (4b); and cercariae (4c). Cercariae may then encyst as metacercariae in the same first intermediate host or leave to penetrate a new second intermediate host (5). Several animals may become the second intermediate host, such as other snails, bivalves, fish, and tadpoles. The definitive host gets infected after eating infected second intermediate hosts (6). The metacercariae excyst in the duodenum (7). Adults then live in the small intestine (8). Image reproduced from the Division of Parasitic Diseases, Centers for Disease Control and Prevention (CDC), Atlanta, GA.

The author would like to acknowledge Paul Chen, BS, ScM (2008) in Genetic Epidemiology, Johns Hopkins University Bloomberg School of Public Health, whose contributions and insights were invaluable for the revision of this article.